Appears to boost residual enmity decay by 5% when struck. Very useful for Ranger's who hover near the hate cap on long-term battles such as Dark Ixion. Obviously you'd macro it in when you take hate, but I didn't think that needed stating.
I think this will be a good idle piece for mages who pull hate with cure and big nukes. we'll just have to see how much it actually decreases to know for sure though.